Tensile strained semiconductor photon emission and detection devices and integrated photonics system

ABSTRACT

Tensile strained germanium is provided that can be sufficiently strained to provide a nearly direct band gap material or a direct band gap material. Compressively stressed or tensile stressed stressor materials in contact with germanium regions induce uniaxial or biaxial tensile strain in the germanium regions. Stressor materials may include silicon nitride or silicon germanium. The resulting strained germanium structure can be used to emit or detect photons including, for example, generating photons within a resonant cavity to provide a laser.

RELATED APPLICATIONS

This is a CONTINUATION of U.S. application Ser. No. 15/000,975, filedJan. 19, 2016, which is a CONTINUATION of U.S. application Ser. No.14/698,759, filed Apr. 28, 2015, now U.S. Pat. No. 9,270,083, which is aCONTINUATION of U.S. application Ser. No. 14/256,758, filed Apr. 18,2014, now U.S. Pat. No. 9,036,672, which is a CONTINUATION of U.S.application Ser. No. 13/209,186, filed Aug. 12, 2011, now U.S. Pat. No.8,731,017, each of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to optical systems that includesemiconductor light emitting devices or semiconductor light detectors.More specifically, the present invention relates to semiconductor lightemitting or detecting devices that incorporate a strained group IVsemiconductor material in an active region.

DESCRIPTION OF THE RELATED ART

There is continuing interest in using group IV semiconductor materialsin photonics systems because of the ease of manufacturing such systemsand the ease of integrating these group IV photonics with circuitry.Silicon, germanium and their alloy are the group IV semiconductors mostfrequently considered for photonics systems. For example, light emissionfrom silicon or within silicon is of great interest. Silicon andgermanium exhibit an indirect bandgap, as does their alloy over its fullrange of compositions. Conventionally these are not efficient materialsfor light emission because the conduction band involved in a directoptical transition is not occupied, so that there are essentially noelectron-hole pairs that can recombine and generate a photon directly,without the additional contribution of another entity such as a latticevibration or impurity.

A cost-effective way to integrate photonic functions with silicon-basedULSI chips such as multi-core processors or leading-edge memory wouldopen the door to far-reaching architecture changes and performanceimprovement for modern computing. One suggested application of thesephotonic functions is to replace some of the intra-chip copperinterconnects within modern ULSI chips, for example for routing datafrom one CPU core to another, where both cores are on the same physicalsilicon chip. At the same time, a practical group IV photonics solutioncould provide extreme cost reduction benefits in manufacturing moreconventional photonic systems.

The principal ways to incorporate photonics with existing CMOS processflows include the following topologically distinct options: i)manufacture the optical components before the transistor; ii)manufacture the optical components after the transistor integration,i.e., either before, within or directly after the metal interconnectlayers; or iii) manufacture an optically-enabled layer using group IVsemiconductors that can be attached to ULSI chips by one of variousmechanisms. The attachment mechanisms may include wafer bonding,co-packaging of several dies next to each other where they arewire-bonded or connected by features in the package, and stacking diesand connecting them, for example using through-silicon vias (TSVs).Using a separate optical layer allows for decoupling of themanufacturing challenges and critical integration steps encountered intransistor and ULSI electrical interconnect manufacturing from thoserequired for the optical layer.

On the other hand, it is advantageous to emit light on-chip to avoidcoupling and alignment issues that otherwise have to be solved. On-chiplight emission is very challenging when using group IV semiconductors asthe optically active, light-emitting material in the optical layer. Theliterature reports light emission in silicon using the Raman effect toconvert externally supplied light of a certain wavelength to a differentwavelength. Light emission using the Raman effect is a low efficiencyprocess.

An optical system or an optical layer typically has several functionalcomponents. An optical layer usually includes a light source, perhapswith an integrated bandwidth filter to select from a broad spectrum thewavelength, i.e., the “color” of light being utilized. The light sourcecan be a laser that emits coherent light or a light emitting diode. Thelight source can be either directly modulated, e.g., by modulating thecurrent through the light source, akin to switching a light bulb on andoff (high and low), or by modulating information onto the “light beam”through a separate component external to the light source, i.e., using amodulator. External modulators are known in the art, including ringmodulators and Mach-Zehnder modulators.

An optical layer usually includes at least one waveguide that can routelight in the form of a continuous wave or in modulated form, i.e., as asignal, from one point to another. Waveguide performance considerationsinclude attenuation, the degree to which light is lost per unit length,e.g., due to light scattering or due to light absorption into thewaveguide or adjacent material. Another important performance metric isthe waveguide's ability to turn guided light into another direction witha small turning radius without significant loss of light. Tight turningradii can be achieved, e.g., by using high confinement waveguides wherethe refractive index of the guide is considerably higher than in thesurrounding volume so that the light wave intensity is mostly carriedinside the waveguide volume. The interplay between turning radius andleakage of the evanescent tail of light intensity outside the waveguideis an important parameter for the design of ring modulators or routingswitches. Tight turns can also be facilitated by means of mirrors forwhich the angle between the direction of the incoming light and thenormal to the mirror surface is substantially the same as between thedirection of the outgoing light and the mirror normal direction. Afurther aspect is the degree to which the waveguide maintains a givenlight polarization.

An optical layer usually includes a routing or a switching element thatreceives light from an incoming waveguide and selects from a number ofoutgoing waveguides one or several that will carry the outgoing light. Amirror can be thought of as a routing element with one incoming and oneoutgoing waveguide. Other examples for these elements include arrayedwaveguide couplers, multi-mode interference couplers and ring-couplers.

An optical layer usually includes a detector that measures the intensityof the incoming light with accuracy and at high speeds. Often detectorsare reverse biased photodiodes. The responsivity and external andinternal quantum efficiency of photodiodes should preferably be high forthe wavelength of light to be detected. Often their speeds are limitedby the RC value, the product between the detector capacitance Gunctioncapacitance and stray capacitance) and the resistance value andcapacitance of the conductors leading to the reverse biased junction.The RC value measures the time within which charge carriers generated atthe detector junction can deliver a detectable current at the electricaldetector terminal, i.e., the external speed of the detector.

An optical layer usually includes drive electronics either on the sameoptical layer or in a separate layer, e.g., in the CMOS chip for whichthe photonics layer provides part of the interconnect.

Future data transmission bandwidth requirements, e.g., between racks inserver farms, from one board to another board, from a processor toelectrical circuit board or to memory will continue to grow into theseveral Tbps data bandwidth range. Current optical components for lightsources, modulators or even detectors cannot operate at thesefrequencies. More specifically, the ability to put information onto acarrier beam either by direct modulation of light sources or by means ofa modulator currently do not exceed frequencies of several tens of Gbps.

Therefore an approach in which multiple light beams (equivalent to anumber of bus lines) are used to transmit the data in parallel isnecessary to get to Tbps system bandwidths. If the light beams carryingthe information have different wavelengths multiple carrier signals canbe transmitted through a single waveguide and couplers. Such a schemecalled wavelength division multiplexing (WDM) is well-known intelecommunications. A multitude of point-to-point connections using thesame or similar wavelengths can be envisioned and waveguides can evencross each other since the light beams do not interact with each other.

It is desirable to build such a WDM system or a network ofpoint-to-point connections within a single optical layer to reduce cost.

Several methods to generate light within an optical layer are known. Onemethod is the hybrid laser, which achieves light amplification byletting some light energy being guided in a silicon waveguide reach orextend into an optically active InP-based multiple quantum well materialin which the light amplification is achieved by electrically pumping theoptically active transitions in the direct bandgap InP-based material.

Another prior art method utilizes a reduction of the direct bandgap ofgermanium which is achieved through the biaxial straining of germanium.The strain occurs because of a mismatch of thermal expansioncoefficients between germanium and the substrate on which the germaniumis deposited in a process step at elevated temperature. Upon loweringthe temperature the germanium becomes biaxially tensile strained to asmall degree, typically less than 0.3%. In this case, the strain is notstrong enough to fully turn germanium into a direct bandgap material andthe energetically smallest transition from the conduction band to thevalence band of germanium continues to be a transition that is notoptically allowed (i.e. it is indirect and involves anotherquasi-particle such as a phonon or lattice vibration.) The predominanceof the indirect band transition is countered by doping an active regionof the light emitting device very strongly n type, so that the states inthe lowest lying conduction band valley are populated. Under a highlevel of electrical injection of carriers into the n+ region, carriers(electrons) spill from the conduction band valley for which an opticaltransition is forbidden (indirect gap) into the energetically slightlyhigher conduction band valley for which the optical transition isallowed (direct gap.) The forbidden transition becomes saturated, andcarriers spill into the more effective direct bandgap transition states.

Where light is generated on-chip, i.e., within the optical layer,optical layers can use homogenous materials or a heterogeneous materialsystem. In a homogeneous material system, light is emitted and detectedin a material that is chemically substantially the same for allcomponents of the system, such as the light source, waveguide,modulator, switch or detector. In a heterogeneous material system thelight is emitted in a material that is chemically different from thewaveguide or the detector material.

SUMMARY OF THE PREFERRED EMBODIMENTS

An aspect of the present invention provides an optical device having agermanium region in contact with a plurality of stressor regions. Theplurality of stressor regions induces tensile strain within thegermanium region. The tensile strain in at least a portion of thegermanium region is sufficient to cause a portion of the germaniumregion to have a direct band gap. A junction is positioned in oradjacent the portion of the germanium region, the junction having afirst side with a first majority carrier type and a second side with asecond majority carrier type. First and second contacts are respectivelycoupled to the first side of the junction and the second side of thejunction.

According to another aspect of the present invention, an optical devicecomprises first and second germanium regions. The first germanium regionis in contact with a first tensile stressor so that the first germaniumregion has biaxial tensile strain in at least a first portion of thefirst germanium region. The second germanium region is in contact with asecond tensile stressor so that the second germanium region has biaxialtensile strain in at least a second portion of the second germaniumoptically active region. Optical elements define an optical path throughthe first and second germanium regions. A junction is positioned in oradjacent the first and second portions of the first and second germaniumregions, the junction having a first side with a first majority carriertype and a second side with a second majority carrier type. First andsecond contacts are respectively coupled to the first side of thejunction and the second side of the junction.

According to another aspect of the present invention, an optical devicecomprises a germanium slab having first and second faces and first andsecond ends and first and second stressor layers on the first and secondfaces. The first and second stressor layers induce biaxial tensilestress within the germanium slab. Optical elements are positioned withrespect to the germanium slab to define an optical path passing throughthe germanium slab.

According to another aspect of the present invention, an optical devicecomprises two or more germanium slabs each having first and second facesand first and second ends and first and second stressor layers on eachof the first and second faces. The first and second stressor layersinduce biaxial tensile stress within respective ones of the two or moregermanium slabs. Optical elements are positioned with respect to thegermanium slabs to define an optical path passing through the two ormore germanium slabs.

According to still another aspect of the present invention, a method ofmaking a semiconductor device comprises providing a substrate having agermanium region and etching openings into the germanium region. Themethod continues by forming silicon germanium within the openings toform a pattern of embedded silicon germanium surrounding a first portionof the germanium region, the silicon germanium regions and the firstportion of the germanium region having in-plane biaxial tensile strain.

Still another aspect of the present invention provides a method ofcommunicating data comprising coupling an electrical signal into anoptical device comprising a first strained semiconductor region togenerate a responsive optical signal. The method continues bytransmitting the responsive optical signal through a waveguidecomprising a second unstrained semiconductor region and coupling theresponsive optical signal into a detector comprising a third strainedsemiconductor region. The first, second and third semiconductor regionscomprise germanium. In a more specific implementation of this aspect,these regions are essentially self-aligned to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a portion of a light emitting ordetecting device that can have a tensile strained active region.

FIG. 2 illustrates the results of a simulation based on the FIG. 1structure that shows a high level of induced strain within a germaniumstrip between embedded silicon germanium stressors.

FIG. 3 illustrates schematically a germanium region surrounded by apattern of embedded stressors such as silicon germanium or siliconnitride, where the embedded stressors have a generally rectangularsurface cross section.

FIG. 4(a) illustrates in perspective a germanium layer with an array ofseparate embedded stressor regions composed of a material under tensilein plane stress such as silicon germanium or silicon nitride, where theembedded stressor regions have a generally rectangular surface crosssection and the stressor regions induce tensile strain in the adjacentgermanium regions.

FIG. 4(b) illustrates in perspective a germanium layer with an array ofseparate embedded stressor regions composed of a material under tensilein plane stress such as silicon germanium or silicon nitride, where theembedded stressor regions have a generally rounded or circular surfacecross section and the stressor regions induce tensile strain in theadjacent germanium regions.

FIG. 4(c) illustrates in perspective a germanium layer with an array ofconnected embedded stressor regions composed of a material under tensilein plane stress such as silicon germanium or silicon nitride, where theembedded stressor material surrounds the periphery of pillar shapedregions of germanium, the germanium pillar regions having a generallyrectangular surface cross section and the stressor material inducing inplane biaxial tensile strain in the adjacent surrounded germaniumregions.

FIG. 4(d) illustrates in perspective a germanium layer with an array ofconnected embedded stressor regions composed of a material under tensilein plane stress such as silicon germanium or silicon nitride, where theembedded stressor material surrounds the periphery of pillar shapedregions of germanium, the overlapping embedded stressor regions having agenerally rounded or, in the limit, circular surface cross section andthe stressor material inducing in plane biaxial tensile strain in theadjacent surrounded germanium regions.

FIG. 4(e) illustrates in perspective a pillar shaped germanium regionwithin embedded stressor regions composed of a material under tensile inplane stress such as silicon germanium or silicon nitride. Theillustrated pillar may be one within an array of like pillar shapedregions of germanium, with the stressor material inducing in planebiaxial tensile strain in the surrounded germanium regions.

FIG. 5 illustrates another implementation of aspects of the invention inwhich tensile strain is created in a fin of germanium through forceimposed by compressive stressor layers on sidewalls of the germaniumfin.

FIG. 6 illustrates a modification of the FIG. 5 strategy in whichbiaxial tensile strain is created in the germanium fin by patterning ofthe compressive stressor layers on the sidewall of the germanium fin.

FIGS. 7(a-b) illustrate using three dimensional simulation a furthermodification of the strategy of FIGS. 5 and 6 in which a plurality ofbiaxially tensile strained germanium fins are provided along an opticalpath.

FIGS. 8(a-b) schematically illustrate a strained germanium strip wheretensile strain is induced by an overlying compressively stressedmaterial and edge relaxation.

FIG. 9 schematically illustrates a strained germanium strip wheretensile strain is induced by upper and lower layers of compressivelystressed material and edge relaxation.

FIG. 10 schematically illustrates a strained germanium strip wherebiaxial tensile strain is induced in the strip by upper and lower layersof compressively stressed material and edge relaxation with cuts throughthe three layers along two axes.

FIG. 11 illustrates a preferred further modification of the FIG. 10strategy that limits internal reflections.

FIG. 12 schematically illustrates in cross section an array of tensilestrained n-type germanium regions epitaxially deposited on a p-typegermanium layer so that the structure can emit or detect photons.

FIG. 13 schematically illustrates in cross section an array of shallowtensile strained n-type germanium regions doped by diffusion from anoverlying layer of doped polysilicon.

FIG. 14 schematically illustrates in cross section an array of epitaxialtensile strained n-type germanium regions formed on an array ofepitaxial p-type germanium regions so that the structure can emit ordetect photons.

FIG. 15 schematically illustrates in cross section an array of tensilestrained p-type germanium regions in contact with an electron emitterlayer so that the structure can emit or detect photons.

FIG. 16 schematically illustrates in cross section an array of tensilestrained p-type germanium regions in contact with an electron emitterlayer so that the structure can emit or detect photons.

FIG. 17 illustrates in schematic cross section an array of tensilestrained p-type germanium regions in lateral contact with n-type silicongermanium regions and an electron emitter layer so that the structurecan emit or detect photons.

FIG. 18 illustrates another strategy, consistent with the structures andprocesses illustrated in FIGS. 12-17, in which one or more discretegermanium pillars are formed and embedded within a continuous silicongermanium stressor.

FIG. 19 schematically illustrates a germanium waveguide coupled with astructure having in-plane strained or biaxially strained germaniumpillars or fins that emit or detect light.

FIG. 20 illustrates schematically a configuration in which a layercontaining tensile strained germanium is coupled with a resonator toprovide a laser structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention include a light emissionor light detection device or method which uses a strained group IVsemiconductor as the active region that emits or detects light. Lighthere is used in its broad sense to incorporate ultraviolet and infraredranges. As one example, an implementation of the present invention mightprovide a semiconductor laser that uses tensile strained germanium as again medium. Most preferably, this specific example may use a germaniumregion biaxially tensile strained to a sufficient extent that at least aportion of the strained germanium region is a direct bandgapsemiconductor.

Certain embodiments of the present invention may use a distinct opticallayer made up of generally homogeneous materials to form variouscomponents of an optical layer including at least one light source, oneor more waveguides, at least one routing or switching element, or atleast one detector. Drive electronics are included, either in theoptical layer or in another layer such as in an associated VLSI chip.For the homogeneous material case, the material constituting thecomponents is of course physically somewhat different, because the useof a homogeneous material system necessitates the local change of someof the optical properties of the material in question, to turn itvariously from light emitting (direct bandgap) semiconductor material,to optically transparent (indirect bandgap) waveguide material or tolight detecting (direct bandgap) semiconductor material.

Most preferably, the variations needed to achieve the desired localoptical properties are created, e.g., through the application ofexternal strain, in particular bi-axial or uniaxial tensile strain. Inaddition, preferred implementations locally alter the electricalproperties of the semiconducting material in question in the usual wayssuch as dopant ion implantation or diffusion to build electrical devicesin the usual manner.

Certain preferred embodiments define an optical layer from a generallyhomogeneous material system for which some of the semiconductor materialin select components either is made to have a fully direct or fullyindirect bandgap. In particularly preferred embodiments, the bandgap inthe emitter or detector is lower than the bandgap of the waveguide, sothat the waveguide is essentially transparent to the photons emitted bythe emitter or detected by the detector. In the case of a waveguideformed of a semiconducting material with an indirect bandgap, leavingthe material of the waveguide unstrained allows the material to retain alarger bandgap than the strained emitter or detector material. Thelarger bandgap in the waveguide together with the indirect nature of thebandgap result in a lower optical transmission loss in the waveguidematerial.

For particularly preferred implementations that use strained germaniumfor active light emission or absorption, the source and the detectorwill be turned from being an indirect semiconductor (and therefore arelatively inefficient optical emission/absorption material) to a moredirect semiconductor (and correspondingly to an efficient opticalemission/absorption material). In stating that the tensile strain in thegermanium causes it to become a more direct band gap semiconductor it ismeant that the tensile strain causes optical transitions correspondingto the direct transition between the conduction band minimum at thegamma point to the valence band to be more probable due to the reducedenergy gap between the conduction band minimum and the valence band atthe gamma point. That is, highly tensile strained germanium exhibits astrong enhancement of luminescence corresponding to the directtransition at the gamma point. This strong enhancement of luminescencecan be exploited to manufacture efficient light emitting devicesincluding light emitting diodes and semiconductor lasers in germanium.

In the case of a semiconducting material with a direct bandgap such asgallium arsenide, the waveguide can be turned into a non-absorbingsemiconductor through application of compressive stress along at leastone axis that increases the bandgap in the waveguide and makes it moretransparent for wavelengths that are emitted from unaltered bulk galliumarsenide. As a result transmission losses through absorption arereduced.

The following describes several illustrative implementations of methodsand devices that can form components of photonic systems with strainedsemiconductor light emitting or detecting elements.

Group IV semiconductors generally exhibit the diamond structure and assuch have the principal directions <100>, <110> and <111>, which arerepresentative of the symmetry of the crystal structure. These axes arenormal to the (100), (110), and (111) lattice planes, respectively.Deformation of the natural equilibrium lattice (distances between atomsand angles between atoms) leads to changes in the band structure. Forexample, to first order, hydrostatic pressure results in a homogeneousvolume compression of a cubic lattice and most commonly to an increaseddirect bandgap. For germanium the effect of uniaxial, biaxial andhydrostatic strain on the band structure has been of scientific interestfor a long time.

The application of biaxial tensile strain within the (100) plane ofgermanium renders the material more direct, i.e., an increase in (100)biaxial strain narrows the direct bandgap more rapidly than the indirectbandgap. Calculations of the band structure of biaxially tensilestrained germanium predicts that the material becomes fully direct atabout 1.9% of strain in the (100) plane. Furthermore, uniaxialdeformation is reported to lead to a direct bangap when applyinguniaxial tension along the <111> direction of germanium. Of course, alarge number of strain orientations and configurations with respect tothe principal directions of a crystal can provide different advantagesfor turning an indirect bandgap material into a more direct bandgapmaterial.

FIG. 1 shows an example in which embedded silicon germanium (SiGe)stressors are used to tensile strain a germanium region (such as a finor stripe) which can be used either for light emission or lightdetection. In a preferred embodiment multiple tensile stressors areembedded in a germanium layer causing a tensile strain in the volume ofgermanium between any two neighboring tensile stressor regions. SiGealloy is a suitable tensile stressor material when grown epitaxiallyinside a recess formed in a germanium surface. SiGe alloy has a crystallattice spacing that is smaller than the crystal lattice spacing ingermanium. Therefore when a thin film of SiGe is grown epitaxially on agermanium surface the SiGe is under tensile in-plane lattice misfitstrain. A tensile strained embedded SiGe stressor induces tensile strainin laterally adjacent germanium. As a more concrete example, FIG. 1illustrates a portion of a germanium layer 10. Two long trenches are cutin the surface of germanium layer 10 and are filled by epitaxialdeposition of SiGe to form embedded stressors 12, 14 on either side ofgermanium region 16. The embedded SiGe stressors 12, 14 on both sides ofthe narrow strip of germanium 16 induce uniaxial tensile strain in thevolume of germanium between the SiGe stressor regions as indicated inFIG. 1. The narrow strip of tensile strained germanium 16 may be used asa light emitting active region of a laser or diode or as a lightdetecting region of, for example, a photodiode. The height or thickness,width and length of these structures are preferably selected to achievedesired levels of strain to achieve the optical functionalityappropriate to the component. Preferably, the dimensions are selectedaccording to the specific implementation and the other elements of thecomponent.

Advanced CMOS technologies frequently include embedded SiGe source ordrain (S/D) regions in the manufacture of high performance p-channelfield effect transistors. In the case of SiGe source drain regionsembedded in a silicon transistor a compressive strain is obtained in thesilicon region between the SiGe stressors. This is the inverse of thestrain condition that arises when SiGe stressors are embedded in agermanium device as described here but the manufacturing processes andtechnologies, design considerations and implementations are verysimilar.

FIG. 2 illustrates a simulation of a structure generally like that shownin FIG. 1, with FIG. 2 showing a cross section perpendicular to thelonger axis of the stripe. The FIG. 2 simulation uses a slightly morecomplicated and practical structure including a buried insulating layer28 between a silicon or other wafer or substrate 20 and a layer ofgermanium 26 in which trenches are formed and subsequently at leastpartially filled with silicon germanium to form tensile stressor regions22 and 24. As shown in the simulation of FIG. 2, high levels of straincan be induced, especially near the upper surface of the germaniumstripe 26.

Alternatively SiGe stressors can be incorporated in a germanium laser orlight emitting diode or photodector in the form of a matrix of multipleembedded regions having depths and widths of approximately 100 nm by 100nm. These dimensions are illustrative and a range of dimensions can beused effectively. The particular stated dimensions are useful for theillustrated configuration. In this strategy, the light-emitting activeregion of a laser (typical size 0.35 to 1.5 micron wide, or even wider,by 2 to tens or even hundreds of microns long, or even longer) orlight-detecting region of a photodetector may consist of many regions ofgermanium with biaxial tensile strain induced along two in-plane axes byadjacent volumes of embedded SiGe. FIG. 3 depicts one element of such amatrix wherein biaxial tensile strain is induced in a central germaniumregion 30 by adjacent SiGe stressor elements 32, 34, 36, 38 formed onfour sides. The stressor elements have a generally rectangular surfacecross section that may, for example, be approximately square. Thestressors are illustrated as silicon germanium, which provides tensilestrain to the adjacent germanium regions. Other stressor materials mightalso be used, such as silicon nitride deposited to provide tensilestress. The strain distribution is generally inhomogeneous and dependenton the geometry and other relevant characteristics such as thecomposition of a silicon germanium stressor. The strain might, forexample be highest around upper side portions of the germanium region.Biaxial strain may be maximal in a central portion of germanium region30.

FIGS. 4(a-e) illustrate how a multitude of germanium elements may bearranged in a matrix so that each element has biaxial tensile strain inat least a part of the element. The germanium elements under biaxialtensile strain may be connected to neighboring germanium elementssimilarly under biaxial strain as shown in FIG. 4(a) and FIG. 4(b).Alternatively the germanium elements may be separated from neighboringgermanium elements by the stressor material as shown in FIG. 4(c) andFIG. 4(d) in which case the separate germanium regions appear aspillars. Although the germanium pillar elements are separated bystressor material in the plane of the wafer, they may of course remainconnected through their bases to a shared or common remaining germaniumlayer or substrate. Furthermore the germanium pillars may have asubstantively square cross sectional profile or may be rounded having inthe limit a circular cross sectional profile or a concave sided pillarprofile as illustrated in FIG. 4(d). FIG. 4(e) shows one of an array ofgermanium regions within embedded stressor regions composed of amaterial under tensile in plane stress such as silicon germanium orsilicon nitride. The illustrated pillar may be one within an array oflike pillar shaped regions of germanium, with the stressor materialinducing in plane biaxial tensile strain in the surrounded germaniumregions.

In particularly preferred implementations of this embodiment, an arrayof strained germanium elements like that illustrated in any view of FIG.4 may be used as the light emitting active region of a laser device orlight detecting region of a photodetector device. The FIG. 4 lightemitting active region consists of multiple at least partially biaxiallytensile strained germanium elements. For the purpose of achieving alasing action it is not necessary that the whole volume of the laseractive region be formed of biaxially tensile strained germanium but itis desirable to maximize the proportion of the laser volume that isbiaxially tensile strained germanium. In addition, it is desirable toinduce sufficient tensile strain in the germanium regions so that theregions become direct bandgap over at least a portion of the regions. Inthe portion of a germanium region with the largest biaxial tensilestrain the direct conduction band (gamma point) minimum is at its lowestenergy. In particular for any portion of germanium with a biaxial strainof greater than about 1.8% to 2.0% the direct gap (at the gamma point)should be smaller than the indirect gap and that portion of thegermanium may be considered a direct gap semiconductor. Under theseconditions free electrons will drift to the portion with the mostbiaxial tensile strain (lowest conduction band energy) and this willcoincide with the portion where the direct optical transitions are morefavorable. As such, even though only a fraction of the germanium volumein a laser may have a biaxial tension sufficient to induce a direct gapbehavior, this may be adequate for practical purposes of stimulatedemission of photons because the direct gap portion is at the same timethe portion of germanium to which free electrons are drawn by the builtin field arising from the reduced conduction band energy level in thatportion. Conversely the less strained portions of germanium that do notturn direct, will remain indirect and not provide light amplification.However light emitted from the highly strained portions having thesmallest and direct bandgap will not be absorbed in the less strainedportions having a larger and indirect bandgap. Although the lessstrained portions of germanium are not expected to contributesignificantly to light emission in the light emitting diode orsemiconductor laser those portions also should not contributesignificantly to losses. It is nevertheless desirable to maximize thefraction of the germanium in the optically active region that is under ahigh level of biaxial tensile strain.

Preferably for this configuration, the silicon germanium stressors arenot doped or are not doped n-type to avoid pinning the germaniumconduction band and to facilitate the described effect of funnelingcarriers into the gain region by laterally varying the conduction band.The embedded stressors could alternatively be formed of silicon nitridedeposited with built-in tensile stress. Methods and tools are well knownin the silicon integrated circuit manufacturing industry for depositingfilms of silicon nitride with built-in tensile stress.

In the embodiment illustrated in FIG. 5, a germanium substrate 22 ispatterned and etched to form a germanium fin structure 52 extendingabove the remaining portion of the substrate 50. Fin structure 52 may,for example, be 0.05 μm in width and 0.15 μm in height. Compressivestressors 54, 56 are formed on the side walls of the germanium fin 52 toimpart uniaxial tensile strain in the germanium fin as representedschematically by the arrows in FIG. 5. Typically, the illustratedstructure is formed by depositing a conformal blanket layer ofcompressively stressed silicon nitride. The stressed silicon nitride isthen preferably etched away from the top of the fins to allow forelectrical contact to the tops of the fins by etching the siliconnitride layer to leave silicon nitride only along the sidewalls. Becausethe initial stress in the sidewall stressors is compressive, thesidewall stressors 54, 56 expand vertically when they relax and inducetensile strain in the germanium fin structure 52.

Methods and tools are well known in the silicon integrated circuitmanufacturing industry for obtaining deposited films of silicon nitridewith built-in compressive stress and methods are known for formingsidewalls of a material such as silicon nitride by deposition andsubsequent anisotropic etch. Sidewall stressors alone will impose auniaxial tensile strain in the germanium fin directed vertically,orthogonal to the plane of the semiconductor workpiece or wafer. Hereagain, the FIG. 5 tensile strained germanium fin active region 52 canact as light emitting region or a light detecting region, depending onthe geometry and subsequent processing. Preferably the original strainin the sidewall stressor structures 54, 56 and the dimensions of the fin52 are appropriate to create sufficient vertical (uniaxial) tensilestrain to cause a portion of the fin active region 52 to have a directband gap. Earlier or subsequent processing can be used to form agenerally horizontal p-n junction in the illustrated fin structure. Forexample, the illustrated fin structure might be formed as p-typematerial. A highly n-type doped polycrystalline germanium layer isformed on an upper surface of the p-type fin 52. Subsequent annealingcauses n-type dopants to diffuse into the fin structure 52 preferably toform a generally horizontal p-n junction. Preferably the p-n junction ispositioned sufficiently adjacent the tensile strained portion of the finstructure 52 to allow the junction to be an efficient photon emitter ordetector. For photon emission, electron hole pairs are generated byflowing current through the junction and photons are emitted throughelectron-hole radiative recombination associated with the preferreddirect band gap. For photon detection, the p-n junction is reversebiased so that photons generate electron hole pairs that separate andare detected as an electrical current through the junction. In photonemission implementations, it is sometimes preferred that end faces ofthe germanium fin are coated with one or more reflective layers toachieve a resonant cavity

In positioning the junction, it is preferred that the junction belocated so that photon absorption (through creation of an electron holepair) or emission (through radiative recombination of an electron holepair) occurs to a sufficient extent in a portion of the germanium thatis tensile strained sufficiently to provide a direct band and to provideefficient photon detection or emission. Alternately, the tensilestrained portion of the germanium preferably includes the lowest bandgap for a direct optical transition when current injection or otherstrategy is used with the reduced band gap to achieve efficientemission. Such an appropriate positioning of a tensile strained regionand a junction or portion of a junction is identified here as adjacentand includes those positionings where the junction portion is coincidentwith a region of locally maximum tensile strain and those where there isan offset between those positions. The possible acceptable size of anoffset is dependent on the level of strain achieved, the application andthe device geometry. This discussion is made specific to thecomparatively simple geometry of FIG. 5 but applies equally to the morecomplicated and other implementations discussed with respect to theother figures. In addition, the positioning and other considerations arealso applicable for those implementations where tensile strain isinsufficient to accomplish direct bandgap transitions. In thosesituations, the principles discussed here apply, but preferably arecombined with desired doping, bias and/or current to achieve sufficientphoton emission or detection to be useful. For any laser application,properly reflective ends of a cavity preferably are provided, losses aremaintained at a suitably low level and sufficient current is provided sothat the cavity provides gain in the manner known in the art. Thisdiscussion references a junction or junctions. In many cases, thejunction will not be a sharp p-n junction but might effectively be ap-i-n junction with p-type and n-type regions positioned on either sideof an active layer that preferably is undoped. A similar pn or p-i-njunction may be used in both light emitting laser diode (or LED) devicesand in photodetector devices.

In a further enhancement of the sidewall stressor method, narrow cutsmay be etched into the silicon nitride compressive stressor layer alongthe length of the fin making the sidewall silicon nitride discontinuousalong the length of the fin. This is illustrated in part in FIG. 6,where a narrow cut 68 has been etched through one of the compressivelystressed silicon nitride sidewall structures to form multiple sidewallcompressive regions 64, 66 that can expand both vertically and laterallyto induce both vertical and horizontal strain components in thegermanium fin 62. At the breaks or cuts in the silicon nitride sidewall,edge relaxation (i.e., relaxation facilitated by expansion orcontraction at the comparatively unconstrained edges of the stressors)causes an additional stress component to be induced in the adjacentgermanium fin directed along the length axis of the fin, as shown inFIG. 6. This configuration induces biaxial tensile components of strainin segments of germanium active region 62 which is desirable formodifying the germanium band structure to reduce the band gap for directoptical transitions, preferably to an extent that the direct opticaltransition is the lowest energy transition. As illustrated in FIG. 6 theadditional vertical cuts in the compressive silicon nitride sidewalllayers may be vertical although the cut lines also may be at a differentangle.

An alternative method to obtain biaxial tensile strain in the germaniumactive element of the laser or photodetector introduces breaks or cutsin both the sidewall stressor elements and the germanium waveguide alongthe length of the fin to better induce biaxial tensile strain componentsin the germanium fin. If cuts are etched into the germanium finwaveguide, the gaps in the germanium along the length axis of the laseror photodetector may be undesirable as they will act as partial mirrorscausing unwanted internal reflections or scattering of light generatedin the laser active region or of light in the photodetector. Thisundesirable behavior may be limited by depositing amorphous germanium inthe gaps. The edge relaxation that occurs when the gaps are etched inthe germanium is sufficient to induce tensile strain along the lengthaxis of the germanium fin waveguide or active region. Subsequentrefilling of the gaps with, for example, amorphous germanium does notremove the tensile strain but does largely remove the dielectricdiscontinuity in the waveguide or active region of the laser orphotodetector along the length axis. That is, the refilling of the gapswith a suitable material such as amorphous or polycrystalline germaniumrestores a continuous optical medium along the longitudinal optical axisof the laser or photodetector active region but with discontinuoustensile strain along the longitudinal optical axis of the laser ordetector active region.

FIG. 7 illustrates a three dimensional simulation of a furthermodification of the strategy of FIGS. 5 and 6. FIG. 7 shows a number offins, each a fin of germanium 72 with a dielectric (or insulating)stressor 74, 76 formed on either side of the germanium fin. Each ofthese fins may be formed in the manner discussed above with respect toFIGS. 5 and 6, including the etching, doping, contact and junctionformation strategies discussed there. Preferably the dielectricstressors are initially formed to have compressive stress that isrelaxed through etching to induce tensile stress in the germanium fins72 between the stressors 74, 76. One suitable stressor is siliconnitride, which can be deposited to have compressive stress that can berelaxed through appropriate etching strategies. As shown in FIG. 7(a),the dielectric stressor layers 74, 76 can fill the gap between adjacentfins 72 and effectively induce a desirable level of biaxial tensilestress in the germanium fins. Simulated biaxial strain in the fins 72 isillustrated in FIG. 7(b) where the stressor regions have been renderedinvisible to reveal contours of biaxial strain in the germanium asevaluated in the major planes of the fins, the lighter contoursindicating greater magnitude of biaxial strain. The array of biaxallytensile strained germanium fins can be positioned so that an opticalpath, for example of a diode, laser diode or photodetector, passesthrough the FIG. 7 structure so that the optical path passes through aplurality of the fins in a direction parallel to the lateral fin faceson which the stressors are formed to induce stress. Alternately, theoptical path of the exemplary diode, laser diode or photodetector passesthrough a plurality of the fins in a direction perpendicular to thelateral fin faces on which the stressor layers are formed to inducestress.

For the germanium fin structures discussed above in exemplary FIGS. 5-7,the fins may have a width (separation between dielectric stressorlayers) of between about 20 nanometers and 100 nanometers and, morepreferably, between about 40 nanometers and 80 nanometers. The finspreferably have a height (as measured above the remaining germaniumlayer adjacent the base of the fin) of less than one micron and, morepreferably, less than 400 nanometers. The fins preferably have a length(as measured laterally along the face of the germanium layer adjacentone of the stressor layers) of less than one micron and, morepreferably, less than 400 nanometers. For the implementations of FIGS.5-7 and other implementations of the structures described here, it ispreferred that the compressively stressed silicon nitride stressormaterial is formed to initially have a stress of greater than twogigapascals and, more preferably, greater than three gigapascals.

FIGS. 8-11 schematically illustrate elastic edge relaxation ofcompressive stressor layers on top or bottom surfaces of a germaniumstripe or slab. Compressive stressor layers on the top surface (or top &bottom surfaces) of a germanium stripe laser or photodetector activeregion are patterned and etched in alignment with the active region.Edge relaxation occurs when a stripe pattern is etched through the topstressor layer, the active layer and, optionally, the bottom stressorlayer. Compressive stress in the stressor layers induces tensile strainin the adjacent germanium active layer.

In a first implementation of this strategy, illustrated in FIG. 8(a), alayer of compressively stressed silicon nitride is deposited on thesurface of a germanium wafer or substrate 80. The process forms a maskon the silicon nitride layer and then etches through the silicon nitridelayer and into the surface of the germanium wafer to form a stripe (orslab) of germanium 82 extending above the remaining portion of the wafer80. The etching through the compressively stressed silicon nitride layerforms the stripe 84 and the etching continues into the substrate,allowing the compressively stressed silicon nitride 84 to relax andinduce tensile strain in at least the upper portion of the stripe 82 ofgermanium. The resulting strained surface region of the stripe 82 can beused to generate photons or to detect them.

In a presently preferred implementation, a host wafer has a germaniumlayer wafer-bonded to the surface of the host wafer. For example, thehost wafer 86 might be a silicon wafer 83 with a surface silicon oxidelayer 85, or a portion of a silicon integrated circuit covered by asilicon oxide layer, and the germanium layer is bonded to the oxidesurface in the well known manner. A compressively stressed stressorlayer is then deposited on the germanium layer. For example,commercially available processes are available to deposit appropriatecompressively stressed silicon nitride layers with a built in, asdeposited stress of greater than two gigapascals or, more preferably,three gigapascals. The FIG. 8(b) structure is again formed by patterningand etching a compressively stressed stressor stripe 84 followed byetching a long stripe of continuous germanium active region 82, stoppingat the surface of the wafer 86 as illustrated in FIG. 8(b). The edgerelaxed surface stressor stripe 84 induces a uniaxial strain in thegermanium stripe 82, as indicated by the arrow in FIG. 8(b). Inalternative embodiments etching does not stop at the surface of wafer 86but rather continues to a small depth into the surface of wafer 86 so asto induce a greater tensile strain in the germanium stripe 82. In otheralternative embodiments etching stops before the surface of the wafer 86is reached so that the germanium stripe 82 is in the form of a pedestalof germanium standing on a layer of unetched germanium.

FIG. 9 illustrates another configuration that can, as compared to theFIG. 8(a)-(b) configurations, provide a greater level of strain to agermanium stripe to better emit or detect photons. The FIG. 9 structureis formed by sequentially depositing a first layer of compressivelystressed material on a host wafer 90, providing a layer of crystallinegermanium, and then depositing a second layer of compressively stressedmaterial on the germanium layer. The first and second layers ofcompressively stressed material may, for example, be compressivelystressed silicon nitride and the deposited materials are selected tofunction as stressors for the germanium layer. Processes for depositingsuch silicon nitride stressor layer are known. The FIG. 9 processcontinues by patterning and etching through the stack of layers to forman upper stressor stripe 94, a germanium stripe 92 that is uniaxiallytensile strained in the direction illustrated by the arrow in FIG. 9.The etching may optionally continue through the compressively stressedsecond layer below the germanium layer to form the lower stressor stripe96. Etching through or at least into the second compressively stressedlayer is preferred as it provides more complete edge relaxation andresults in a greater level of uniaxial tensile strain. The germaniumstripe 92 preferably is between 0.04 and 1.0 microns wide and ispreferably tensile strained to a sufficient extent that at leastportions of the germanium stripe 92 have a direct bandgap.

A number of factors influence the level of tensile strain within thegermanium layer, including the thickness of the germanium layer, therespective thicknesses of the upper and lower stressor layers and thelevel of compressive stress within the upper and lower stressor layers.Tensile strain also varies by the separation between the edge andwhatever portion of the germanium layer is being considered. Thenon-uniform distribution of strain is true of all of the structuresdiscussed or illustrated here. It is preferred that the tensile strainwithin this or other germanium regions discussed here (and elsewhereincluding above with respect to FIGS. 5-7) be adjusted to provideefficient photon emission or detection. It should nevertheless beappreciated that the structures and strategies described here can beused advantageously when lower levels of tensile strain are achieved,even when the material exhibits an indirect bandgap and photon emissionrelies on high carrier injection levels.

In other embodiments the patterning and etching is done to makeadditional cuts or breaks into the germanium stripe and the adjacentcompressively stressed (as deposited) stressor layers along the lengthaxis of the stripe, breaking the active region stripe into shortersegments of length typically in the range 0.04 to 1.0 microns. One suchembodiment is illustrated in FIG. 10. The FIG. 10 implementation is likethe embodiments illustrated in FIG. 9, except that when the stripe ispatterned and etched, further patterning and etching is performed toopen edges and a gap 108 between portions of the germanium stripe 102,104.

The gap 108 allows the first and second stressor (upper and lower)stripe portions to elastically relax through edge relaxation to moreefficiently induce tensile strain within the germanium stripe portions102, 104. The edge relaxation that occurs when the gaps are etched inthe germanium is sufficient to induce tensile strain in the germaniumalong the length axis of the germanium rib waveguide or active region.The longitudinal tensile strain is in addition to the transverse tensilestrain induced along the width axis of the germanium stripe. Thisconfiguration induces biaxial tensile components of strain in segmentsof germanium active region which is desirable for modifying thegermanium band structure to reduce the band gap for direct transitions.As illustrated in FIG. 10 the additional vertical cuts in thecompressive silicon nitride sidewall layers may be vertical or the cutlines may be at a different angle than vertical.

Immediately after creating the gaps, the stressor stripe portions relaxlaterally and induce tensile strain within the remaining portions of thegermanium stripe. Subsequent refilling of the gaps, for example, withamorphous or polycrystalline germanium does not remove the tensilestrain in the remaining portions of germanium stripe, but does largelyremove the dielectric discontinuity in the waveguide or active region ofthe laser or photodetector between different portions of the germaniumstripe along the length (or optical) axis of the device. This isillustrated schematically in FIG. 11, where amorphous or polycrystallinegermanium 116, 118 is deposited within the gaps such as gap 108 betweenportions of the germanium stripe 102, 104. Refilling the gaps with asuitable material restores a continuous optical medium in the segmentedgermanium stripe of the laser or photodetector active region that hasdiscontinuous tensile strain.

In certain embodiments of the tensile strained germanium laser diode orphotodetector diode, the material 116, 118 that refills the gaps betweensegments of the germanium active region may be doped and used as anelectrical conductor in the diode. In a preferred embodiment the refillmaterial is n+ doped polycrystalline-SiGe and acts as the electronemitter in the laser diode, emitting electrons laterally into p-typedoped strained germanium regions 102, 104. Doping polycrystallinegermanium or silicon germanium n-type during deposition is well knownand readily accomplished.

FIGS. 12-17 show various alternatives for forming an electrical junctionfor a system that includes desirably tensile strained germanium. Thestandard mode of operation of a semiconductor laser requires stimulatedemission of photons to occur in an active region formed at the junctionof two material regions, one region providing a source of holes and theother region a source of electrons such that radiative recombination ofthe holes and electrons occurs in the vicinity of the junction of thetwo regions. The two material regions are typically p-type and n-typesemiconductor regions respectively and form a pn junction where theymeet. If a region that is neither strongly p-type nor strongly n-type ispresent between the n-type and p-type regions, the junction is referredto as a pin junction where the nominally undoped region is considered“intrinsic.” In some implementations, an undoped layer or layers isprovided between p-type and n-type layers to form the desired junction.

In the strained germanium laser, the region of efficient radiativerecombination of carriers (electrons and holes) preferably coincideswith the region of maximum biaxial tensile strain in the germanium. Inpreferred embodiments of the device the region of maximum currentdensity through the pn junction is to the greatest degree possiblecoincident with the region of maximum biaxial or uniaxial tensile strainin the germanium. The plane of the pn or pin junction may bepredominantly parallel to the wafer surface or may be predominantlyperpendicular to the wafer surface. Given the difficulties in dopinggermanium n-type by activation of implanted donor species, it may bepreferred that the n-type germanium region be formed in an n-type dopedstate either in the starting wafer (which may be bulk germanium orgermanium-on-insulator) or in the epitaxial germanium layer asappropriate. The p-type germanium region may be formed by implantationand activation of an acceptor species such as boron or by epitaxialgrowth of a p-type germanium region on top of the n-type germanium.Alternatively the junction may be formed by epitaxial growth methodsstarting with a p-type germanium bulk wafer or germanium-on-insulatorwafer and growing an n-type germanium layer to form an epitaxialjunction.

The electron emitter may be formed of a material different fromcrystalline germanium. n+ doped regions are difficult to manufacture ingermanium due to poor activation of implanted donors in germanium. Adeposited electron emitter material may be preferred where the emittermaterial may be any of: n+ in situ doped amorphous or polycrystallinegermanium; n+ in situ doped amorphous or polycrystalline silicon oramorphous or polycrystalline silicon germanium; a low work functionmetal with work function less than 4.3 electron Volts; or a low workfunction metal with an interfacial dielectric layer between the metaland the germanium, the dielectric layer being thin enough to allow anelectron current to flow through it. In embodiments where the germaniumlayer is on an insulator such as buried oxide (BOX), the contact to the(typically p-type) germanium preferably is a separate contact.

In the embodiment of the strained germanium laser, diode orphotodetector with embedded SiGe stressors, illustrated in FIG. 4 and inFIGS. 12-17, an epitaxial pn or pin junction may be formed in thegermanium before the SiGe stressor regions are formed. In this case theSiGe stressor regions may be undoped, doped p-type or doped n-type.Separate electrical contacts are made to the n-type and p-type regionsof germanium.

Insulating silicon oxide regions may be formed self-aligned to thesilicon germanium (SiGe) stressor regions by the following-describedmethod. The desired pattern of a matrix of embedded regions is definedby lithography and dry (plasma) etching in a layer of silicon nitridewhich is deposited on the germanium. The germanium is etched where it isnot covered by silicon nitride to create recesses in the germaniumsurface. The recesses are filled with epitaxial SiGe alloy by a methodsuch as chemical vapor deposition (CVD). If the CVD epitaxial process isselective, SiGe is grown epitaxially only in the recesses and not on topof the silicon nitride. If the CVD process is non-selective, the SiGe isdeposited over all exposed surfaces in which case a subsequentplanarization process such as chemical-mechanical polishing (CMP) isused to remove SiGe from silicon nitride surfaces leaving SiGe only inthe mask openings and within the recesses in the germanium structure. Atthis stage in the process with the silicon nitride mask still coveringgermanium surfaces, an oxidation process is applied such that exposedsurfaces of recessed (embedded) SiGe are oxidized. This grows aninsulating thin film of silicon oxide or silicon germanium oxide on topof and self-aligned to the SiGe regions. The silicon nitride is thenremoved using a selective wet etch and the top surfaces of the biaxiallystrained germanium elements are exposed. At the same time, removing thesilicon nitride allows a more complete transfer of stress from the SiGeregions which are biaxially tensile strained in the plane of the waferto the laterally adjacent germanium regions which also become biaxiallytensile strained in the plane of the wafer.

FIG. 12 shows one implementation for forming a tensile strainedgermanium structure for a photon emitter or photon detector. For ease ofdiscussion, this structure will be described with reference to agermanium diode laser utilizing in-plane biaxial tensile strain and,most preferably, a direct bandgap optical transition. Those of ordinaryskill will understand that the present structure could be implemented asa simple light emitting diode rather than as a laser and that, withproper bias and amplifier, the illustrated structure could be used as adetector such as a photodiode. As discussed above, appropriate junctionsinclude pn junctions and pin junctions. FIG. 12 starts as illustratedwith a p-type germanium substrate 120, which might alternately be ap-type layer on an insulating layer such as a buried oxide (BOX) layer.Still further, the illustrated p-type germanium layer might bepositioned on or over silicon including silicon circuitry or opticalelements such as waveguides based on silicon or silicon oxidestructures. These various possibilities for the germanium substrate aresimilar for the other implementations illustrated in FIGS. 12-18 and soare not repeated in the discussion of those illustrations.

The FIG. 12 implementation preferably forms a structure like thatillustrated in FIG. 4 with the strain and optical properties discussedabove with respect to FIG. 4. In the FIG. 12 implementation, a layer 124of intrinsic or lightly doped n-type germanium is epitaxially depositedon the p-type germanium substrate 120, with the n-type doping preferablyaccomplished in situ during deposition. A mask, such as a siliconnitride mask, is formed over the intrinsic or lightly doped n-typeepitaxial germanium layer, with openings in the mask defining a matrixpattern such as a checkerboard pattern above the intrinsic or lightlydoped n-type epitaxial germanium layer 124. Dimensions of the regionscan vary while achieving the desired tensile strain and might, forexample, be generally square in top cross section and might beapproximately 0.04 to 1.0 microns on a side. Etching through thegermanium layer 124 and preferably into the p-type substrate 120proceeds to form a corresponding array of openings or recesses in thegermanium structure. Silicon germanium regions 126 preferably are formedepitaxially by selective chemical vapor deposition within the openingsin the p-type germanium layer 120 and intrinsic or lightly doped n-typegermanium layer 124 as defined by openings in the silicon nitride mask.In this illustration, the silicon germanium regions may be undoped orintrinsic. If the silicon germanium regions 126 are not depositedselectively or if otherwise preferred, chemical mechanical polishing maybe performed to remove excess silicon germanium. Subsequently, theexposed silicon germanium preferably is oxidized by exposing thesurfaces to an oxidizing environment to form the insulating silicongermanium oxide structures 128. Preferably, the silicon nitride masklayer is then removed.

As discussed above, formation of the silicon germanium regions adjacentand around the germanium regions creates biaxially tensile strainedsilicon germanium regions, which in turn, induce in-plane biaxialtensile strain in the germanium regions 124. Preferably the biaxialtensile strain is sufficient to cause these portions of the germaniumregions to be direct bandgap so that they can be pumped and efficientlyproduce optical output. These biaxially tensile strained germaniumregions 124 can then be used as components of a laser gain region. Thesilicon germanium regions 126 consequently are also within the lasergain region and do not contribute to the generation of optical output.Contacts are formed to the intrinsic or lightly doped n-type germaniumregions. For example, a layer 122 of n-type doped amorphous orpolycrystalline silicon germanium or n-type doped germanium could beprovided to form a contact to the n-type germanium regions 124.Similarly, a region 129 of p-type doped amorphous or polycrystallinesilicon germanium can be provided to form a contact to the substrate orbase p-type germanium region 120 or other methods such as metal plugscan be used. Further processing preferably is performed to providemirrors that define a resonator or laser cavity encompassing at least aportion of the strained germanium so that the biaxially tensile strainedgermanium regions can provide laser action.

FIG. 13 illustrates a different process for providing a structuregenerally like that illustrated in FIGS. 4 and 12. Like structures areindicated with the same numbers in FIG. 13 as are used in FIG. 12. Here,the process begins with a p-type germanium substrate or layer 120, whichis patterned and etched by forming a silicon nitride mask with a matrixof openings followed by dry (plasma) etching to form recesses. Silicongermanium regions 126 are grown, preferably by selective chemical vapordeposition, to form in-plane tensile strained regions with remainingportions of the germanium substrate 120 extending up between the silicongermanium regions 126. The portions of the germanium substrate 120underlying the silicon germanium regions are compressively strainedin-plane. The exposed surfaces of the silicon germanium regions 126 areoxidized and then the silicon nitride mask is removed. Strongly n-typedoped amorphous, polycrystalline or crystalline silicon or silicongermanium or germanium is deposited and patterned appropriately to formthe layer structure 139 illustrated in FIG. 13. Doped layer 139 is incontact with the surface of the germanium layer 120 in the definedmatrix pattern between the silicon germanium regions 126. The oxidizedsilicon germanium layers 128 separate the silicon germanium regions 126from the doped layer 139 so that the dopants from the doped layer 139 donot diffuse into the silicon germanium regions 126. The structure isheated, for example by rapid thermal annealing, to diffuse n-typedopants from the strongly n-type doped layer 139 into the surface of thegermanium 120 to form shallow n-type regions 134 in a matrix pattern,also forming junctions in that same matrix pattern. The resultingstructure can be incorporated in a diode, laser or detector as describedabove. In addition, the resulting structure will have a straindistribution and therefore will be able to generate broad band emission.For laser applications, mirrors can be used to select the desiredwavelength from that broad band emission, advantageously providing arange of possible gain and output wavelengths. For detectors or diodes,filters can be formed adjacent the emission or detection regions toselect emission or detection wavelengths.

FIG. 14 illustrates another variation on the tensile strained germaniumstructure that can be used for emitting or detecting photons. The FIG.14 structure and processes are similar to those illustrated anddescribed with respect to FIG. 12 and so the detailed description is notrepeated. Structures that are substantially similar between the FIGS. 12and 14 are identified by the same reference numbers. The FIG. 14structure is formed on a highly p-type doped germanium substrate 140. Alayer of epitaxial p-type doped germanium 142 is deposited on the moreheavily doped p-type germanium substrate. Subsequent processing follows,for example, as described above with respect to FIG. 12. The resultingFIG. 14 structure has properties similar to the structure of FIG. 12 butwith a more conductive p-type substrate so that there is less seriesresistance and the photon emission and detection device is generallymore efficient.

FIG. 15 shows another implementation of the FIG. 4 in-plane biaxiallytensile strained germanium structure. The substrate 150 is p-typegermanium that is patterned with a silicon nitride or other mask todefine a matrix of stressor positions. Etching into the p-type germaniumsubstrate 150 forms a matrix of recesses in which silicon germanium isdeposited epitaxially, preferably using selective chemical vapordeposition, forming biaxially tensile strained silicon germanium 156.The silicon germanium regions 156 are oxidized to form silicon germaniumoxide regions 158. Surface portions 155 of the p-type germaniumsubstrate are consequently biaxially tensile strained through the forceapplied from the surrounding silicon germanium regions 156, in themanner discussed above. Removing the silicon nitride mask allows formore complete strain transfer. In the FIG. 15 process, an electronemitter material 159 is deposited and patterned as illustrated. Adeposited electron emitter material may be preferred to provide moreconductivity or more design flexibility. Appropriate emitter materialmay be any of: n+ in situ doped amorphous or polycrystalline germanium;n+ in situ doped amorphous or polycrystalline silicon or amorphous orpolycrystalline silicon germanium; a low work function metal; or a lowwork function metal with an interfacial dielectric layer that is thinenough to be electrically conductive between the metal and thegermanium.

In the FIG. 15 configuration, the preferred radiative recombination foran emitter such as a laser occurs primarily in the upper, biaxiallytensile strained portions 155 of the p-type germanium substrate.

FIG. 16 shows a modification of the FIG. 15 configuration, in whichoxide regions are not formed over the silicon germanium regions 156 andthe electron emitter layer 169 is formed in direct contact with thesilicon germanium regions 156. Other aspects of the FIG. 16configuration are the same as those discussed above with respect to FIG.15 and its process.

FIG. 17 provides a further modification of the FIG. 16 structure inwhich the silicon germanium regions 176 are doped during deposition sothat the silicon germanium regions 176 are n-type. In thisconfiguration, electrons can be injected from the electron emitter layer169 above the tensile strained germanium regions 155 and laterally fromsilicon germanium regions 176 around the regions 155. The n-type silicongermanium regions 176 increase the electron emission into the radiativerecombination regions, particularly into the higher strain regions ofthe strained germanium regions 155 and thereby increase the efficiencyof the photon emission process. For detector implementations, thestructure illustrated in FIG. 17 provides more junction area to collectphoton-generated electron hole pairs, providing a more efficientdetector structure. The structures illustrated in FIGS. 12-17 allow forphoton emission and detection using similar structures, making itgenerally simpler to construct emitters such as diodes or lasers as wellas detectors such as photodiodes on the same substrate (wafer) using atleast some common process steps.

Preferred embodiments of the illustrations of FIGS. 4 and 12-17 positionfour silicon germanium embedded stressor regions around an in-planebiaxially strained germanium region. In an array including a number ofbiaxially strained germanium regions according to such embodiments,embedded silicon germanium regions can be adjacent to multiple germaniumregions. The tensile strained silicon germanium regions on at least twosides and preferably four sides of the germanium region preferablyinduce biaxial strain in the germanium region. In some implementations,the silicon germanium regions are not substantially connected toadjacent (nearest neighbor) silicon germanium regions. The silicongermanium stressor regions may have a square, rectangular, rounded orcircular lateral cross section. In particularly preferredimplementations of these (non-continuous stressor) embodiments, thewidth (or equivalently, length) of each lateral dimension of thegermanium region between opposed embedded silicon germanium stressorregions is less than 400 nanometers and, more preferably, less than 100nanometers. Preferably, the tensile silicon germanium regions have asilicon composition of between 20% and 100% silicon and, morepreferably, between 40% silicon and 60% silicon. The preferredembodiments of the present invention are implemented with asubstantially 100% germanium region (which, given the depositionenvironments, may include silicon to a measurable extent), but it shouldbe understood that the germanium regions could be implemented in afuture implementation with some extent of silicon or carbon and bewithin the teachings of the present invention.

FIG. 18 illustrates a preferred embodiment of the structures andprocesses illustrated in FIGS. 12-17, in which one or more discretegermanium pillars are formed and embedded within a subsequentlydeposited silicon germanium stressor layer. In general, the FIG. 18structure is formed on a germanium substrate by providing a mask throughlithography or any other patterning method to define the locations andextent of isolated germanium pillars. Patterning and etching defines thelateral extent of the germanium pillars. Etching depth defines pillarheight. Pillars are isolated in the lateral sense but are preferably notisolated from the underlying germanium substrate so that adjacentpillars share a common germanium region or substrate. The pillars might,for example, have a height above the remaining germanium region orsubstrate of between about 20 nanometers and 400 nanometers or, morepreferably, between about 40 nanometers and 100 nanometers. Thegermanium pillars may have a square, rectangular, rounded or circularlateral cross section and preferably have a lateral dimension of morethan 20 nanometers and less than 200 nanometers and, more preferably,have a lateral dimension of between 30 nanometers and 100 nanometers.Preferably the germanium pillars are formed in a regular array such as a“checkerboard” pattern in which the pillars are spaced apart by uniformx and y separations.

After forming the array of germanium pillars, manufacture of the FIG. 18structure proceeds by depositing a layer of silicon germanium around thepillars. The silicon germanium is deposited on the surface of thegermanium substrate so that the silicon germanium will be in a tensilestrained state. Preferably, the tensile strained silicon germanium layerhas a silicon composition of between 20% and 100% silicon and, morepreferably, between about 40% silicon and 60% silicon. As discussedabove with respect to FIGS. 12-17, the silicon germanium depositionprocess may be done selectively or may be performed and then excesssilicon germanium can be removed through, for example, chemicalmechanical polishing. As is also described above, the tensile strainedsilicon germanium layer induces lateral, biaxial tensile stress withinthe germanium pillars, preferably to an extent to cause the directoptical transition to be the lowest band gap of the biaxially strainedgermanium pillars. The more specific manufacturing and structuralstrategies illustrated in and described with respect to FIGS. 12-17 canbe implemented in the geometry and arrangement of germanium pillarsillustrated in FIG. 18. In FIG. 18, which represents a part of theactive region of a strained germanium laser according to one embodimentof the invention, multiple pillar regions 182 are formed in a patternedarray by etching into germanium layer 180 and filling the etchedtrenches with a tensile stressed material such as epitaxial silicongermanium as represented by region 184. In the particular examplerepresented here, the trenches surrounding each germanium pillar aredeliberately merged such that the tensile stressed trench fill materialforms one continuous region 184.

It is, of course, also possible to combine the overlying stressor layer(e.g., in plane biaxially compressively stressed silicon nitride) withembedded stressors (e.g., in plane biaxially tensile stressed silicongermanium) to biaxially tensile strained germanium. Preferably theoverlying stressor layer has openings where the embedded stressors areformed so that it covers the germanium regions to be strained. Alsopreferably the overlying stressor is removed after the strainedgermanium is formed.

The invention provides the possibility, in a further refinement, tointentionally position the optically active, highly emitting strainedgermanium pillars or fins (for example regions 182 in FIG. 18) atspecifically determined locations along the principal optical axis ofthe laser resonant cavity corresponding to a spacing equal to one halfwavelength of the resonant optical mode of the cavity. That is, one orseveral rows of optically active germanium elements may bepreferentially spaced at intervals of one half wavelength of the desiredoptical mode of the laser cavity. This allows an optimization of lightamplification in the cavity and minimization of reabsorption (opticalloss) by eliminating (avoiding) strained germanium regions at locationsthat do not contribute to light amplification and that would otherwiseonly add to electrical and optical energy losses.

Fabrication of light emitting diodes or lasers or photodetectors in abody of tensile strained semiconductor (e.g., germanium) enables a wholephotonics system including light emitters, optical couplers, waveguidesand photodetectors to be combined and integrated within the same layerof semiconductor (e.g., germanium). Where light emission or detection isrequired the semiconductor is differentiated by locally tensilestraining the semiconductor (e.g., germanium), the straining causing theoptical semiconductor (e.g., germanium) band gap to be narrowed and thesemiconductor's band gap to become more direct. Where light emission ordetection is not required the semiconductor is not intentionally tensilestrained and the band gap remains wide and indirect. Examples of opticalcomponents in which light emission or detection is not required includewaveguides and optical couplers, and preferably the semiconductor (e.g.,germanium) regions corresponding to such circuit components are notintentionally strained. In a preferred embodiment the semiconductor isgermanium and the germanium is locally biaxially tensile strained inlocations where an active optoelectronic device such as a laser, lightemitting diode or photodetector is fabricated. In a preferred embodimentthe biaxial tensile strain is equal to or greater than about 2% in asufficient proportion of the germanium region within an activeoptoelectronic device to achieve the desired active optoelectronicdevice functionality, be it photon emission or photon detection. Activeoptoelectronic device regions preferably are differentiated from passiveoptoelectronic device regions primarily by the degree of tensile strainand less so by a difference in the elemental composition of the activematerial. Conventional photonic integrated circuits solely or primarilyuse changes in elemental composition to differentiate active frompassive optoelectronic devices.

In an example of a conventional indium phosphide based photonicintegrated circuit, the passive waveguide is a layer of indium phosphideand the active components include an active layer comprising indiumgallium arsenide or indium gallium arsenide-indium phosphide multiplequantum wells. Light is emitted by the indium gallium arsenide which isan optically active, direct band gap semiconductor material as a resultof its chemical composition, not as a result of strain within thematerial. Here the light is emitted by a material that is not the samematerial as the waveguide material. In general, the light emittingmaterial is added to the waveguide material by epitaxial growth or by abonding method wherever a laser is fabricated. Preferred aspects of thepresent invention facilitate use of the same material as an emitter ordetector and as the waveguide by altering the material's opticalproperties at least in part by imposing a strain.

In general, assembling optical networks consisting of light emitters,modulators, waveguides, and detectors together requires alignment of thecomponents in three dimensions and at angles to a very high degree ofcontrol and precision. A typical figure of merit in aligning the opticalaxis of a waveguide with that of a detector is to obtain at least 50 to80% of transmission, which for Gaussian beam profiles requires analignment of better than approximately 10% of the cross sectiondimension of the waveguide, which is on the order of 0.1 um. This istypically done with a lot of effort, using active or passive alignmentstrategies. As a result, yield and cost issues make optical networkingcomponents much more expensive than semiconductor integrated circuits.There are large and ongoing efforts to find cost-effective, integratedassembly solutions. Aspects of the present invention can be used tolimit assembly and alignment issues. A typical process flow to buildoptical aspects of a system implementing aspects of this invention drawsfrom steps already used in the manufacture of integrated circuits. Usingsuch existing technology offers the possibility that well-establishedmethods for yield improvement and cost reduction can be applied tooptical interconnect, communication or other systems.

Manufacturing an integrated optical (photonics) system in a singlesemiconductor layer draws from established front-end line processescurrently used in leading-edge microelectronics manufacturing: wetcleans, epitaxy of group IV elements (silicon, germanium or theiralloy), deposition of dielectric films, patterning by way of lithographyand subtraction of material through suitable wet and dry etches,followed by CMP, and miscellaneous steps to dope and make electricalcontact to the electrical components of the optical system. Bonding,hetero-epitaxy of IIIN or IINI compound semiconductors or depositingnon-group IV materials, be they crystalline or non-crystalline, maysupplement aspects of the optical systems described here, but are notessential to achieve integrated photonic systems. A preferred method forfabricating an integrated photonic system on a semiconductor wafer mayrequire little alignment of optical components other than selfalignment.

In a preferred embodiment the invention provides an optical system inwhich at least some and in particularly preferred implementations allcomponents including emitter, waveguide, and detector are made fromsubstantially the same element (e.g., germanium) where the material isstrained locally and selectively in such a way that it becomes opticallyactive with a band structure that corresponds to the band structure of adirect gap semiconductor only where required by the system designer,i.e., within the gain medium of lasers, within light emitting diodes orwithin photodetectors. Preferably, a waveguide defined in part laterallyby low dielectric constant materials, an emitter such as a laser havinga gain region including one or more biaxially strained germaniumregions, and a detector such as a photodiode comprising one or morebiaxially strained germanium regions, with the waveguide, active regionsof the emitter and the detector self-aligned to one another.

In the embodiments depicted in FIGS. 12-17, as previously discussed, theemitter layer overlying the strained germanium regions may be dopedamorphous or polycrystalline germanium or silicon or silicon germaniumalloy. In such embodiments it is advantageous to select a thickness andcross-sectional geometry for the emitter layer such that the lightintensity profile (mode field pattern) is contained partially in thegermanium layer and partially in the emitter layer with the aim ofmaximizing the overlap of the light intensity profile and the volume ofoptically active biaxially strained germanium pillar or fin regions. Thebiaxially strained regions of germanium may be in the upper part of thegermanium waveguide structure. Those of ordinary skill in the art candesign the overall structure to position the optical mode with themaximum optical intensity coincident with the most highly strainedregions of germanium. By this means the light amplification bystimulated emission in the strained germanium regions is optimized. Thispreferred embodiment is represented in FIG. 19 where the laser is formedin a rib waveguide 192 that is etched into a germanium layer 190. Therib is clad on adjacent sides by regions of low index dielectricmaterial such as silicon oxide, 194 and 196. Silicon germanium tensilestressor regions are indicated as 197 and the columns of germaniumbetween the silicon germanium stressors are biaxially strained andoptically active providing radiative recombination and stimulatedemission of light. Emitter region 198 is formed over the lasing regionof the germanium rib in a pattern that is optionally overlapping ontothe low index regions 194 and 196. Within the body of the laser theoptical intensity profile is preferably centered on the highly biaxiallystrained regions of the germanium pillars within the rib as indicated bythe dashed line 199 which represents the mode field pattern within thegermanium laser. In a further refinement the emitter region 198 isbeveled or tapered down in thickness along the axis of the germanium ribin the direction leading away from the lasing region such that theoptical intensity profile (mode field pattern) is repositioned back intothe body of the germanium rib waveguide. In this case, thepolycrystalline emitter material does not overlie the germaniumwaveguide except where an optically active device such as a laser orphotodetector exists.

It will be appreciated that the structure illustrated in FIG. 19 canaccommodate the other emitter structures illustrated in FIGS. 4 and12-17 within the general region indicated as 199 in FIG. 19 and withsimilar operation and advantage. Also, the structure generallyillustrated in FIG. 19 can also be used to provide a detector. Opticalsignals are generated in an emitter or laser region 199 and propagatethrough the wedge structure and into the rib waveguide 192. Theseoptical signals may be generated by a driver circuit in a siliconprocessor or memory circuit as part of an electrical to optical bustransaction so that the optical signals carry data from a processor ormemory circuit. The optical signals propagate through the rib waveguide192 to a near or remote detector location where the optical signals canbe converted to electrical signals for further processing in aprocessor, storage in a memory or other desired process. A detector maybe coupled to a rib waveguide through a wedge such as the wedge 198illustrated in FIG. 19 and into a region with fins or pillars ofbiaxially tensile strained germanium configured as a detector asillustrated in FIGS. 4 and 12-17 and as discussed above. Preferably,active regions of each of the emitter an detector are self-aligned withthe rib waveguide and with each other.

Circuitry within a processor can be coupled to circuitry in a spacedapart or remote portion of the processor by providing an optical planesuch as a layer of germanium. Driver circuitry in the processor outputsa set of data in parallel to a matched array of emitters such as lasers.The lasers might each have the configuration as illustrated in FIG. 19and produce optical outputs modulated with the output of the drivercircuitry that is coupled into a corresponding array of rib waveguides.Signals transmitted in parallel through the corresponding array of ribwaveguides are provided to a corresponding array of detectors havingbiaxially tensile strained germanium fins or pillars as discussed above.The outputs of the array of detectors are provided to driver circuitswhich provide the retrieved signal to an electrical bus that distributesthe signals within the processor.

FIG. 20 shows another configuration for a laser incorporating one of thestrained germanium gain structures discussed above. As illustrated, astrained germanium structure 202 such as any of those discussed abovethat preferably has sufficiently tensile strained germanium presentthrough at least a portion of the structure 202 is provided in contactwith an optical structure 200. For example, the optical structure 200may be a waveguide or an optical cavity for a laser with mirrors 204,206 formed on opposite surfaces of the optical structure 200. Theoptical structure might, for example, be a silicon waveguide, a siliconoxide waveguide or other appropriate structure for a laser cavity. Themirrors 204, 206 on either end of the optical structure, for example,might be distributed Bragg reflectors, the structure and manufacture ofwhich are well known. In the illustrated configuration, one of morelaser modes can couple into the strained germanium portions of thestructure 202 to be amplified by the gain of the region. The couplingbetween structures 200 and 202 may, for example, be evanescent coupling.Preferably sufficient gain is achieved to provide gain to the lasercavity through modes coupling to the adjacent gain medium. Otherconfigurations for the laser structure can be used, including those thathave mirrors formed directly on the strained germanium structure.Various mirrors, including reflective or partially reflective surfacescan be used, as is known in the art. A similar strategy can be used toprovide evanescent coupling between a waveguide and a photodiodestructure as illustrated and discussed above to provide an effectivedetector for guided optical signals.

The present invention has been described in terms of certain preferredembodiments. Those of ordinary skill in the art will appreciate thatvarious modifications and alterations could be made to the specificpreferred embodiments described here without varying from the teachingsof the present invention. Consequently, the present invention is notintended to be limited to the specific preferred embodiments describedhere but instead the present invention is to be defined by the appendedclaims.

What is claimed is:
 1. A method, comprising: patterning and etching agermanium layer to form a germanium fin extending above a substrateregion; forming compressive stressors on side walls of the germanium finto impart uniaxial tensile strain in the germanium fin in a directionorthogonal to a plane defined by said substrate region, said compressivestressors formed by depositing a conformal blanket layer ofcompressively stressed silicon nitride over the germanium fin and thenetching the deposited silicon nitride from a top region of the germaniumfin, thereby exposing a top surface of the germanium fin.
 2. The methodof claim 1, wherein the substrate region comprises a remaining portionof the germanium layer.
 3. The method of claim 1, wherein the germaniumlayer is a layer on an insulator.
 4. The method of claim 1, furthercomprising etching one or more cuts into at least one of the siliconnitride compressive stressors along a length of the germanium fin,making the at least one silicon nitride compressive stressordiscontinuous along the length of the germanium fin.
 5. The method ofclaim 4, wherein at least one of the one or more cuts extends throughthe germanium fin.
 6. The method of claim 1, further comprising formingan n-type doped polycrystalline semiconductor layer on the top surfaceof the germanium fin and performing an anneal, thereby diffusing n-typedopants into the germanium fin.
 7. The method of claim 6, wherein then-type doped polycrystalline semiconductor layer comprisespolycrystalline germanium.
 8. A semiconductor device, comprising atensile strained germanium fin extending above a portion of a substrate,and side wall silicon nitride layers abutting respective side walls ofthe germanium fin, wherein a top region of the germanium fin includes anexposed top surface of the germanium fin, and tensile strain in thegermanium fin is orthogonal to a plane defined by the substrate.
 9. Thesemiconductor device of claim 8, wherein the substrate comprises agermanium substrate.
 10. The semiconductor device of claim 8, whereinthe germanium fin is 0.05 μm in width and 0.15 μm in height.
 11. Thesemiconductor device of claim 8, wherein at least one of the siliconnitride layers includes one or more gaps along a length of the germaniumfin, making the at least one silicon nitride layer discontinuous alongthe length of the germanium fin.
 12. The semiconductor device of claim8, wherein at least one of the gaps extends through the germanium fin.12. The semiconductor device of claim 8, wherein the germanium fincomprises a p-type material.
 13. The semiconductor device of claim 8,wherein the germanium fin includes a p-n junction adjacent a portion ofthe fin under tensile strain.
 14. A semiconductor structure, comprisinga germanium fin extending above a portion of a substrate, andcompressively stressed side wall dielectric layers abutting respectiveside walls of the germanium fin, wherein a top region of the germaniumfin includes an exposed top surface of the germanium fin, the side walldielectric layers include one or more discontinuities along theirrespective lengths, and strain in the germanium fin is biaxial to aplane defined by the portion of the germanium substrate in regions ofthe germanium fin.
 15. The semiconductor structure of claim 14, whereinthe substrate comprises germanium.
 16. The semiconductor structure ofclaim 14, wherein the dielectric layers comprise silicon nitride. 17.The semiconductor structure of claim 14, further comprising anelectrical contact to the exposed top surface of the germanium fin. 18.The semiconductor structure of claim 14, wherein the substrate isdisposed on an oxide.
 19. The semiconductor structure of claim 14,wherein the germanium fin includes discontinuities along its length. 20.The semiconductor structure of claim 19, wherein the discontinuities inthe germanium fin align with the discontinuities along the respectivelengths of the side wall dielectric layers.